Chapter 3: Nebular Hypothesis

Question 1

What are particles?

Answer 1

Electrons, protons, neutrons

these are the building blocks of atoms

Question 2

What are elements?

Answer 2

a substance that cannot be broken down to anything simpler by any chemical means

the building blocks of elements are atoms

Question 3

What is an atom? What is it made of? (define protons, neutrons, electrons, where they are in the atom)

Answer 3

atoms are the building blocks of elements

Protons (Positively charged) - always at least one in nucleus
Neutrons (Neutral/No Charge) - atom may or may not have them in nucleus
Electrons (Negatively Charged) - surround the nucleus

Question 4

What is a neutral atom vs. an ion

Answer 4

a neutral atom is not charged - most atoms

Ion: a charged atom
Negative charge = extra electron(s)
Positive charge = deficiency of electrons

Question 5

How are neurtal compounds usually formed

Answer 5

the combination of positively charged ions and negatively charged ions

Question 6

Define Isotopes

Answer 6

Isoptopes are atoms that have the same number of protons but a different number of neutrons.

all atoms of a particular elemenet have the same number of protons (by definition) but may contaon different numbers of neutrons in nucleus

Question 7

What is Mass Number?

Answer 7

Mass Number is how we identify different isotopes of an element

Mass Number = Number of Protons + Number of Neutrons

Question 8

Lithium has 2 stable isotopes found in nature

1. What does this mean?
2. What are the 2 isotopes?

Answer 8

Lithium has 2 stable isotopes that are found in nature - other unstable isotopes could be made in a lab by shooting a stream of particles at otherwise stable atoms

Lithium-6
Lithium-7

Lithium has 3 protons, so lithium 6 must have 3 neutrons (3+3) and lithium 7 must have 4 neutrons (3+4)

Question 9

How to denote an isotope?

Answer 9

Isotopes are defined by their mass number (sum of protons and neutrons)

1. Write name of element with a dash and then its mass number
2. Write the mass number as a subscript before the elements abbreviation

Ie. Lithium has 3 protons (always)

Lithium-6
* an isotope with a mass number of 6 (3 protons + 3 neutrons)
Lithium-7
* is an isotope with a mass number of 7, meaning it must have 4 neutrons because 3+4=7

Another way to denote isotopes is to write a subscript of its mass number and then the element's abbreviation

Question 10

What are the isotopes of Hydrogen?

Answer 10

Hydrogen-1 and Hydrogen-2 are stable isotopes found in nature

Hydrogen-3 an unstable isotope found in trace amounts in nature
* unstable - breaks down (decays) rather quickly.
* produced in nature as a byproduct of cosmic rays interacting with other atoms in Earth’s atmosphere or surface waters, can also be made in labs
* Half Life: 1/2 of any amount of mass you collect of it will break down in exactly 12.32 years

Question 11

What is Half-Life?

Answer 11

• the amount of time it will take for 1/2 of a radioactive substance to break down
• doesn’t matter what the mass is - will take same amount of time regardless of how much of the elements you have - because rate of break down is constant
• Every radioactive isotope has a haf-life

Question 12

Define Radioactivity

Answer 12

Radioactivity: The spontaneous breakdown of unstable atoms (ie. unstable isotopes) → produces energy and other particles

Question 13

Define Fission

Answer 13

Heavy atom → radioactive breakdown → 2 lighter atoms + energy
- a radioactive process

Question 14

Define Fusion

Answer 14

2 Lighter Atoms → combine → 1 heavy atom + energy
- NOT classified as a radioactive process because the heavy atom that is produced is not always radioactive, it can be stable
- Vast amounts of energy can be produced (ie. hydrogen bomb)

Question 15

Define the periodic table of elements and describe how it is arranged

Answer 15

Periodic table of elements shows all known elements arranged by increasing atomic number (increasing atomic weight)

Atomic Number: Number of protons in nucleous (atomic weight)

Question 16

Define atomic number

Answer 16

atomic number is the number of protons in the nucleus of an elements’ atoms.
More neutrons = heavier
therefore lower atomic numbers indicate lighter elements and higher atomic numbers indicate heavier elements.

periodic table is arranged from lowest to highest atomic number

Question 17

Name and descrbe the first 3 elements on the periodic table.

How were they produced?

Answer 17

1. Hydrogen
2. Helium
3. Lithium

These are the 3 lightest elements (have the 3 lowest atomic numbers – least amount of protons)

They are the first 3 elements ever produced.

They are special because they are the ONLY elements produced by the energy of the Big Bang

Question 19

Describe what caused the formation of elements other than the first 3

Answer 19

1. Nuclear fusion reactions in stars
* Really big stars (sun is too small) have enough energy to synthesise (by fusion) elements up to and including Iron (Fe) - Atomic number 26

2. Supernovae
* Elements higher than atomic number 26 (iron Fe) had to be produced by events with enormous energy
* The explosion of a big star releases so much heat and pressure that all the heavy elements can be produced
* Since we have all these elements inside us and around us, we were all made from the stuff of supernovae

Question 20

What is a supernova

Answer 20

• the explosive death of a huge star
• explosion is virtually instantaneous
• the bright effect can sometimes be seen for weeks, during day and night

Question 21

What is a nebula?

Answer 21

• a cloud of interstellar gas
• HUGE but LOW desnity !

Question 22

Crab Nebula: What was it formed from? When and by whom was it observed?

Answer 22

• In 1054 Chinese astronomers observed the explosion (supernova) of a huge star, which led to the formation of the Crab Nebula
• the Crab Nebula is a famous example of a Nebula formed by a supernova
• records in China say the light from it allowed people to read newspaper at midnight

Question 23

How can Nebulas be formed from Supernovae?

Answer 23

some types of nebulas, specifically supernova remnants, are formed from supernovas.

When a massive star reaches the end of its life, it undergoes a supernova explosion, releasing a tremendous amount of energy and ejecting its outer layers into space.

This ejected material forms a nebula, which is called a supernova remnant.

A famous example is the Crab Nebula

Question 24

What is the Nebular Hypothesis?

Another Name for it?

Answer 24

Nebular Hypothesis AKA Solar Nebular Disc Model
* The Nebular Hypothesis attempts to explain how our solar system was formed.

• States that the solar system formed over a relatively brief period of time and all as a unit, and it formed from a Nebula around 4.6 billion years ago

Collapse of the Nebula: A large, diffuse cloud of gas and dust (mostly hydrogen and helium) began to collapse under its own gravity– this means all the particles were attracted together by the culmulative gravitational effects of all the other particles in the cloud
* This could have been triggered by a nearby supernova or other external forces.

Formation of the Protosun: As the cloud collapsed, it began to spin and flatten into a rotating disk. Most of the material accumulated in the center, forming what would become the Sun.

Planet Formation: In the surrounding disk, small particles of dust and gas began to stick together through processes like accretion, forming planetesimals (small, solid objects). These planetesimals continued to collide and merge, gradually forming larger bodies, eventually leading to the creation of planets, moons, and other objects.

Clearing of the Nebula: As the Sun ignited and began nuclear fusion, strong solar winds pushed away much of the remaining gas and dust, leaving behind the planets and other bodies that now make up the solar system.

Question 25

What does it mean for a nebula to contract “under it’s own self-gravity”

Answer 25

all the particples are attracted together by the cululative gravitational effects of all the other particles in the cloud

Question 26

Compare the density and size of the original Nebula cloud and our solar system now. What does this illustrate?

Answer 26

Original Cloud:
* might have been about a light year in diameter.
* very low density - perhaps only a few thousand atoms in every cubic centimeter

Solar System Now:
* diameter out to the orbit of pluto is ~17 light minutes
* the air around us contains some ten million trillion atoms per cubic centimeter

This illustrates the pronounced shrinkage and flattening of the original cloud

Question 27

Do all Nebulas collapse? Why did our Nebula collapse?

Answer 27

Not any Nebula will just collapse under its self-gravity and form a solar system - we see lots of gas clouds that show no indication of doing so

There is no evidence for what triggered the collapse of our Nebula.
The best explanation is that a pocket of the cloud began to contract because it was recoiling from the shock wave produced from a nearby supernova

Question 28

Describe the Collapse of Our Nebula in steps.

How long did this process take?

Answer 28

1. The best explanation for what triggered the collapse is that a part of the cloud contracted because it was recoiling from the shock wave of a nearby supernova
2. The collapsed region increased in density
3. Atoms were colliding vigorously, generating heat in the process
4. There was a local collapse to that part of the coud
5. This accelerated the collapse
6. Short Lived Increase in temperature from the heat generated by atoms colliding - most heat radiated out into space → temperature is as cool as originally was
7. Eventually, particles so densely packed that heat cannot escape → heat builds up
   —- (Any thermal energy (radiation) that is emitted from the hot central regions would be blocked by the surrounding dense layer of matter)
   —- particles near the centre are moving at very high speeds (gas is very hot) meaning they can provide enough sustaining pressure to prevent any further collapse
8. Fusion reactions can start to take place → generates even more energy
9. A star is born (our sun)

This process happened in less than100,000 years

Question 29

How much nuclear output from the sun each second (not entirely constant)

What type of reactions are these?

What do these reactions produce?

When will the Sun run out of fuel for these reactions?

Answer 29

The sun converts ~600 million tons of hydrogen nuclei into helium nuclei per second, althought this rate is not entirely constant.

These are fusion reactions.

These fusion reactions convert part of the mass of these atoms (~4 million tons) into energy, and release an enormous amount of heat and light energy into the Solar System

Sun loses 4 million tons of mass each second – the sun will run out of hydrogen fuel in roughly 5 billion years.

Question 30

What will happen to the sun when it runs out of hydrogen fuel?

Answer 30

It will turn into a red giant and destroy most planets, including earth

Question 31

which 2 events far into the future will affect our solar system?

Answer 31

1. The Andromeda Galaxy will collide with Milky way in about 3 billion years – we do not know what this will do to our solar system
2. Our sun will run out of hydrogen fuel in about 5 billion years, will expand into a red giant, destroying most planets including earth

Question 32

Define Momentum
Define Angular Momentum
Define Conservation of Angular Momentum

Answer 32

Momentum: a measure of an object’s tendency to move at a constant speed along a straight path

Angular Momentum: a measure of the amound of spin (or orbital motion) or an object

Conservation of Angular Momentum:If no outside force acts on a spinning (or orbiting) object, the object’s angular momentum will not change with time, no matter how the objects interacts with other objects.

Question 33

What defines the ‘sense of rotation’ of the original nebular cloud and its angular momentum?

Answer 33

Some atoms are moving one way, some are moving another way.
When all these motions are added up there is likely some slight cumulative motion in some direction – this defines the angular momentum

Question 34

What led to the flattening of the Nebula Cloud? What does this explain about our solar system?

Answer 34

• the original cloud likely had some angular momentum
• therefore it is inevitable that the cloud will spin faster and faster
• with this sense of rotation, as it contracts, it accelerated the spin
• just like a figure skater pulling their arms in to spin faster
  The inccreasingly rapid spin of orbiting material leads to it’s flattening out
  This explains why our solar system is flat, wnad why the planets all originally moved in the same direction as they orbitted the sun

Question 35

Describe the temperature gradient of the flattened nebula

Answer 35

the flattened nebula had a temperature gradient – very hot near the center and cooler in the outer regions

Question 36

Describe the composition of the nebula.
What the composition affected by the collapse or flattening?

Answer 36

The nebula had a homogeneous composition both before and after the collapse and flattening.
This means the same mix of chemcials and elements existed throughout.

• this means the heavier atoms would NOT have fallen faster towards the protosun than light atoms

Question 37

Why are there clouds in the sky but not at ground level?

Explain how this relates to and draws an analogy to the formation of planets in early solar system

Answer 37

The answer is NOT becasue of more water vapour - in general humidity at low altitudes is same as up high.

It is becasue of the temperature – it is cool enough at higher altitudes for water droplets to condense and form large clouds

Water vapour goes through the dew point termperature

The dew point is the temperature at which air becomes saturated with water vapor and condensation occurs, forming these droplets. When air at a certain altitude cools down to or below its dew point, clouds form at that altitude.

the reason for clouds forming at high altitudes is because it is cooler up there, not because there is more water vapour up there (the formation depends on temperature not composition) - similar idea for early solar system – compositional differences of inner and outer planets were due to temperature gradient.

Question 38

Describe how the temperature gradient of the flattened nebula affected the condensation of elements, and how this affected the compositional differences between inner and outer planets.

Answer 38

At cooler outer regions:
* cool enough that everything condensed (even light elements like hydrogen)

At hotter inner regions:
* only refractory elements (those resistant to heat, like metals and silicates) could condense (and freeze)
* They condensed to metal oxides, alloys of nicket and iron, etc.
* These quickly grew to the size of pebbles
* volatile elements like hydrogen, water vapour, which boil off at low temperatures, remained completely gaseous in the hot inner regions

This led to compositional diffferences between inner and outer planets
Inner planets (Mercury, Venus, Earth, Mars): Formed from heavier elements like metals and silicates.
Outer planets (Jupiter, Saturn, etc.): Formed from both heavy and light elements, including gases like hydrogen.

Question 39

Explain why the random jiggling of atoms nor the occasional collisions from other particles were NOT able to break the bonds between refractory elements.

Answer 39

Refractory elements (those that do not readily respond to heat) were able to condense and freeze at all temperatures in the gradient.
As their atoms and molecules collided, they stuck together and formed strong chemical bonds.
The random jiggling and occasional collisions from other particles were not energetic enough to break them apart.

Question 40

What caused the difference in composition of inner vs. outer planets:
temperature gradient or compositional gradient?

Answer 40

There was no compositional gradient - composition throughout the cloud remained homegenous before and after the collapse and flattening
* Therefore, differences in composition of planets were NOT caused by heavier elements being more abundant near the sun

The compositional difference in inner vs. outer planets is due to the temperature gradient.

• Outer regions were cool enough for everything to condense
• inner regions were hot: so ONLY refractory (heavier) elements that were resistant to heat condensed - light elements could not condense and therefore never accumulated into planets

Question 41

Define and describe the process of accretion.
What played a key role in accelerating the process of accretion?

Answer 41

Accretion:
* describes the process of particles coming together to form larger bodies, first pebbles, then planetesimals, then protoplanets, then planets.
Gravitational attraction played a key role in accelerating the process of accretion, as larger objects exerted stronger gravitational forces, pulling in smaller objects

Particles → Pebbles:
* Small particles (grains of refractory materials) began to stick together, growing into pebbles.
* These pebbles collided gently (due to their shared direction of orbit) and stuck together, gradually forming larger objects, such as stones and boulders.

Pebbles → Planetestimals:
* Over time, these pebbles grew into planetesimals (small planet-like bodies), ranging from 10 to 100 km across - there are still trillions of these

Planetestimals →Protoplanets
Planetesimals continued to collide and merge, growing into protoplanets, which eventually formed the planets we know today.

Question 42

We know the temperature gradient prevented light elements from condensing at inner regions. Thus, inner planets are lacking in hydrogen and helium. What happened to the leftover gases?

Answer 42

The Magic Broom: Refers to the hypothesis that these extra gases were swept out of the inner solar system toward the end of planet formation
* Process not entirely understood therefore given name “magic broom” , which is unfair considering there is strong evidence for it

Question 43

What evidence do we have for the Magic Broom?

Answer 43

Solar Wind:
* The Sun produces a stream of charged particles (mostly electrons) called the solar wind, which flows through the Solar System at several hundred kilometers per second.
* This likely played a role in pushing leftover gases out of the inner Solar System.

T Tauri Stars:
* These young stars (similar to the Sun when it was forming) exhibit strong winds that could blow away surrounding gas.

Planetary Atmospheres:
* Computer simulations suggest that the atmospheres we see around the rocky inner planets (including Earth) are not the original ones, meaning early atmospheres were likely swept away
* New atmospheres for these planets were formed from outgassing - particularly, creating a secondary atmosphere from release of volatile substances from volcanoes
* The atmosphere of earth has now changed even more with appearance of different life forms etc.

For all these reasons, we believe the unused gases n the inner parts of the Solar System were swept out into space.

Question 44

What is solar wind?

What is the typical speed of solar winds flowing throughout the solar system?

What phenomenon do solar winds give rise to on Earth?

Answer 44

Solar Wind:
* a stream of charged particles (plasma - mostly electron) that are ejected from the upper atmosphere of the Sun
* this is a continuous phenomenon
* flows through the solar system at hundreds of km per second

• Northern lights are caused by the interaction of solar wind with the Earth’s magnetic field

Question 45

What is a solar flare? How can they affect Earth?

Answer 45

Solar Flare
a brief eruption of intense high-energy radiation from the sun’s surface, associated with sunspots and causing electromagnetic disturbances on the earth, as with radio frequency communications and power line transmissions.

when there is a flare or eruption on the Sun, the solar wind increases and the interaction of solar wind with Earth’s magnetic field cause** Northern Lights**

Question 46

What has changed Earth’s atmosphere over time?

Answer 46

computer simulations show that Earths atmosphere is not the original one. This is becasue:

• original atmosphere likley swept away by strong solar winds (magic broom)
• Outgassing - the release of volatile substances from volcanoes - likely created a secondary atmosphere
• Atmosphere continued to change with the emergence of diffrent forms of life

Question 47

Summarize the nebular hypothesis (formation of planets)

Answer 47

Solar system started as a large, low density cloud of interstellar gas
1. Nebular Collapse:
* possibly caused by shock wave of nearby supernova - no evidence fro what triggered it.
* part of cloud contracts, local collapse of part of cloud which accelerates collapse, particles colliding generating heat and energy
* eventually, particles so densely packed that heat cannot escape - heat in the hot central regions builds up and is blocked from escaping by the surrounding dense layer of matter
* particles near centre are moving fast enough to sustain pressure and prevent further collapse.
* fusion reactions can begin to take place
* a star is born at the centre (our sun)

Cloud flattening:
* Due to conservation of angular momentum, forming a disk.
* there was likely some angular momentum in original cloud when you sum the directions of everything moving
* this angular momentum would accelerate as the cloud collapses, which caused it to flatten

Condensation:
* All elements were able to condense in outer regions
* Only refractory (heavier) elements were able to condense near the sun, lighter elements remained gaseous

Accretion:
* Small particles stuck together to form pebbles, then planetesimals then protoplanets and full-sized planets.
* particles collided gently due to orbiting in the same direction
* gravitational force helped with this process, larger objects had stronger grav. force which helped them attract smaller objects.

Magic Broom
* Solar winds and other factors swept out unused gases (those light elements that did not condense) from the inner Solar System into space

Question 48

What happened to the “leftovers” from the planet formation process?

Answer 48

• gas was swept into space by solar winds
• leftover condensed elements became asteroids (metal/rock) and comets (mostly ice)

Question 49

What predictions are made by the nebular hypothesis?

Answer 49

1. Planetary Bombardment:

• in early solar system, there must have been large numbers of planetisimals orbiting and colliding with the growing protoplanets.
• Since plantesimals joined protoplanets, the number of planetisimals would have decreased
• Therefore, the bombardment rate should have been very high early on and decreased as time passed.
• Bombardment rate: number of collisions

Evidence:
* the surface of the moon shows a lot of early bombardment that decreased over time

2. Collisions and Their Effects:
The model suggests that there were significant collisions between large bodies late in the formation of the Solar System, which can explain:
Uranus’s Tilt:
Uranus is tipped on its side, possibly due to a major collision with another large object.
Venus’s Retrograde Rotation:
Venus rotates in the opposite direction to most other planets, likely due to a collision.
Formation of the Moon:
The current theory is that Earth collided with a Mars-sized body, which led to the formation of the Moon.
Mercury’s Orbital Inclination:
Mercury’s orbital inclination varies chaotically, possibly due to past chaotic collisions.